There are various kinds of reasons in which a receiving picture becomes deteriorated by various propagation troubles when receiving a television broadcast. The ghost is known as one of major causes deteriorating the television picture quality.
The ghost appears as a faint image at a position out of original picture in the television screen, and which is made by a multiple path channel by a reflecting body such as a building, mountain, and airplane. The multiple path channel is also made by a cause such as a mismatching of impedance in a cable system such as a local video network.
The ghost is generally divided into a preghost and a proximity ghost and a postghost of a long ghost in accordance with delay time on the basis of main signal. The preghost is a ghost in which a signal of strongest magnitude reached later than a directly transferred signal is made to a main signal whereby be produced.
The proximity ghost is within 1-2 .mu.s in delay time, and generally it is not seen as a divided picture, but gives much injury to the main picture. The long ghost is a case that the delay time is more than 25 .mu.s, and its attenuation constant is less than the proximity ghost. These ghosts are the problem to be solved in order to obtain a better picture quality as far as a ground wave system broadcasting and a cable broadcasting are present, and particularly a ghost cancelling function is indispensable in a high quality television system such as EDTV and HDTV.
The ghost canceling system which is presently used or in researching is a method for transferring by inserting a ghost cancellation reference GCR signal for the ghost cancellation during a vertical flyback (i.e., a blanking interval) time period, and a chirp signal, a pseudonoise sequence and the like are already known.
In order to effectively cancel the ghost, it is done if a filtering confirming a transferring channel and becoming a reciprocal of its transferring function is made. In cancelling a ghost by finding out a transferring channel, it is indispensable to use a ghost cancellation reference signal which is a kind of training signal. And, generally a common principle is utilized in which a main signal is delayed and this signal is coincided with the ghost signal on a time axis.
The delayed main signal is so attenuated that its amplitude becomes same as an amplitude of the ghost signal. This signal is converted to generate a signal known as a pseudoghost signal. For a generation of the pseudoghost signal, a presence and a delay degree or a position on time of the ghost signal against the main signal have to be discriminated. This is made by sensing a characteristic of video signal component of publicly known characteristic called as generally a training signal. In case that a video signal is injured by a ghost signal, since a corresponding ghost of the training signal would be appeared during aforementioned interval, the ghost can be easily detected and analyzed. A typical training signal used for the ghost detection and offsetting can be inserted within a vertical flyback time period of a time being not used.
In a television system using a pseudonoise sequence as a ghost cancellation reference signal, a pseudonoise sequence generator memorized with a pseudonoise sequence is provided to a transmitting section and becomes to generate a training signal. This training signal is inserted to one line during the vertical flyback canceling section whereby becomes to transfer to a receiving section of the television system. When a ghost signal is mixed to the television signal, the receiving section of the television system becomes to trace a time position of the ghost signal. The traced ghost signal is made to be an inverse channel at a transversal filter, and be coupled with a video signal mixed with the ghost signal and becomes to offset the ghost.
However, the television ghost canceling device using a pseudonoise sequence as this as a training signal, since the pseudonoise sequence is a binary sequence, in case when a noise is present in the transmitting channel, an error variance value of a ghost channel characteristic evaluation becomes to be increased as much as 2N/(N+1)(N is a constant) times than the noise variance value. And, in case of having a self cross-correlation being a T=0 by a comparison of sequence being N in length in accordance with a characteristic of the noise sequence, a peak value being N is appeared, but when having a self cross-correlation having a delay being T=0, an offset of the ghost canceling means becomes produced.
Further, since the ghost is canceled by using a period sequence as a ghost cancellation reference signal, an output of the cross-correlating means computing the ghost channel characteristic can not clearly discriminate the preghost, postghost and proximity ghost because of a periodic property of the reference signal. Accordingly, there is a problem that a ghost cancelling extent has to be less than a sequence length. This can be solved by sending a sequence being different in length one another with a transfer of the reference signal and discriminating the preghost and postghost by utilizing these two sequences in a receiving set, but in this case also, there is a problem which is rot possible to cancel the postghost distanced away than the sequence length.